Look into a multi bulb digital ballast instead of a dimming one
With the multi bulb ballast you can fit a 400w or 600w or a 1000w HPS or MH bulb depending on requirements at the time.
Using bulbs of the desired wattage's is more efficient and gives longer bulb life then using dimming ballasts.
I use a few of the Lumitek multi bulb ballasts although so far just with 600 bulbs but its nice to know I can move up to a 1000w if I feel the need or even down to 400w for economy etc.
I suggest to do some price comparisons and feedback searches to weed out any problem models your considering.